The present invention is related to a sealing apparatus for chemical delivery systems and, more particularly, to the process of introducing materials under pressure into the interior of rotating tubular members, wherein the pressure within the tubular member is monitored and controlled.
The following discussion deals with the manufacture of optical preforms of rod shape, but it is to be understood that the principles of the present invention are applicable to other, different applications involving, generally, chemical delivery systems wherein the chemicals in gaseous or vapor form are delivered under pressure.
In the modified chemical vapor deposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027 issued in the names of J. B. MacChesney et al. on Aug. 12, 1980, and assigned to Bell Telephone Laboratories, Inc., a rotating glass starter tube has the chemical vapor introduced into the interior thereof, and reaction of the chemicals with the tube takes place within a constantly traversing hot zone. A moving torch heats the glass tube from the outside as the precursor gases are passed therethrough, causing deposition of submicron-sized particles on the inside surface of the tube. The torch is moved along the longitudinal axis of the tube in a plurality of passes to build up layer upon layer of particle lading glass to provide a pre-form tube. Once a sufficient number of layers have been deposited, the pre-form tube is heated, generally to a higher temperature, by the moving torch to cause the tube to collapse to yield a pre-form rod, as it is often called. The delivery system of the reactant gases to the starter tube interior is generally through a fixed metallic hollow tube connected to the source or sources of the gases. It is desirable that the space between the exterior surface of the delivery tube and the interior surface of the glass starter tube be sealed so that the critical gases do not leak out of the starter tube.
In the current method of manufacture, the apparatus which ensures sealed delivery of the deposition chemicals in the gases is a combination of a rotary union element, a structure of holding and sealing the starter tube, and a secondary face seal assembly for routing of purge gases through the structure. This is a complex apparatus that requires frequent maintenance. Existing systems also have the disadvantage of having inherently larger cavities for the accumulation of dead zones of flow, and a tendency to create particle contamination from the rotary union and face seal system. Inasmuch as the chemical delivery system is stationary, the current means of achieving delivery is via the rotary union, featuring a transition of the chemicals from a stationary pipe to a rotary pipe or to the inside of a supply coupling. The chemicals being delivered are at a pressure greater than atmospheric, and the face seal properties are the only restriction to the release of the chemicals to the atmosphere. The rotary union and secondary face seals generate a large quantity of particles from wear, and contribute to the contamination of the coupling. As the seals wear down, excessive leakage occurs both to the atmosphere and into the product (starter tube), resulting in lost product and requiring system maintenance. The complexity of the components involved requires skilled maintenance being performed using requalification through test of the system. Both material and labor costs are, consequently, high.
During the fabrication process, after sufficient deposition has taken place, the preform tube is collapsed to form a pre-form rod, as discussed in the preceding. The general process for collapsing the tube includes reducing the rate of traverse of the heat source creating the hot zone in the tube, thereby increasing the temperature of the hot zone until the tube commences to collapse, being in a semi-molten state in the hot zone. Collapsing is generally started at the end of the starter tube remote from the distal end of the gas delivery tube. Final collapse is generally performed under low gas flow conditions, and, as the tube collapses, its interior volume decreases, resulting in an increase of gas pressure in the remaining tube volume. Too great an increase in the pressure can cause a ballooning of the tube to the extent that it no longer is a uniform diameter. In addition, the increased pressure can cause increased leakage where internal seals are used. Heretofore in the prior art it has been difficult to monitor and control the pressure within the tube to the extent that ballooning and/or leakage can be minimized.
The present invention comprises an arrangement for monitoring the pressure within the starter tube and, through appropriate measuring and control, varying the pressure to prevent ballooning during tube collapse, although it can be used with other rotating tube arrangements.
In a preferred embodiment of the invention, a stationary chemical delivery tube has a sealing member of the type disclosed in the aforementioned related Mueller applications affixed adjacent the delivery end of the tube which forms a seal with the interior wall of the rotating starter tube. The seal between the tube interior and the atmosphere is maintained by the physical properties of the sealing member. In accordance with the invention, a coaxial tube surrounds the delivery tube, and has an interior diameter greater than the outer diameter of the delivery tube to create a passageway therebetween. The means by which the sealing member is affixed to the delivery tube has a passageway which serves to place the interior of the starter tube in communication with the passageway between the two tubes. The coaxial tube, and hence, the passageway extends along the length of the delivery tube to an externally mounted modified T connection wherein the gas within the passageway is applied to a pressure measuring device which measures the gas pressure within the passageway and, hence, the pressure within the starter tube. The T connection is connected to both a vacuum or low pressure source and to a pressurized source of gases used in the vapor deposition process, both under control of the pressure measuring means. This provides an arrangement for controlling leakage of the supply chemicals to the atmosphere and a controlled leakage of atmosphere into the starter tube. Thus, higher or lower pressure within the starter tube can be monitored and controlled.
The present invention, as described in the foregoing, is readily adaptable to the use of a buffer zone, the advantages of which are pointed out in the aforementioned Mueller application. Thus, a second embodiment of the invention, a modified seal coupling member is mounted on the coaxial tube and is sealed to the starter tube by means of an external seal. The coupling member has a buffer gas input port which communicates with the buffer gas chamber formed therein and a low pressure or vacuum port which also communicates with the chamber. The open end of the starter tube also communicates with the chamber, thereby forming a buffer zone between the internal seal within the starter tube and the external seal between the coupling member and the exterior of the starter tube. The coupling member is mounted on the coaxial tube and preferably, although not necessarily, is mounted to the tailstock (or headstock) of the MCVD lathe for support. Thus, with this embodiment of the invention, a buffer zone is created for recirculation of buffer gases.
In still another embodiment of the invention, the coupling member is modified to have a multi-ported central buffer gas feed into the buffer zone, with the feed member projecting into the starter tube, and with the internal sealing member mounted on the distal end thereof. The principles and features of the present invention will be more readily apparent from the following detailed description, read in conjunction with the drawings.